1. Field of the Invention
This invention relates to improvements in heat treating alkaline raw materials in a kiln. In particular, a rotary kiln plant and process are disclosed for burning cement raw materal.
2. Description of the Prior Art
Alkaline substances are often present in raw materials used in the manufacture of cement. In rotary kiln plants for such manufacture, the manufacturing process often includes a step of preheating of the raw materials in a preheater which receives its heat from the exhaust gases from the kiln. This enables a reduction in the processes taking place in the kiln itself, so that the kiln dimensions can be equally reduced.
The exhaust gases developed in the kiln often contain alkali vapours, which for example may be in the form of sulphates and carbonates. These vapours are expelled from the kiln and are condensed on passage through the preheater with the consequent formation of cakings and accumulations of the alkaline substances on the walls and in the various stages of the preheater. Such formations often necessitate periodically stopping the operation of the entire plant for the purpose of cleaning the preheater and the connection between the kiln and the preheater.
Various ways of overcoming this problem have been tried in the past. One proposed method consists of causing a relatively early condensation of the alkaline vapours in the lowermost stages of the preheater, so that the vapours are precipitated on dust particles contained in the smoke gases and are returned as alkaline dust to the kiln. The alkaline dust is then passed out of the system through valve gates, cyclones or filters provided in or associated with the preheater. However, this method suffers from the drawback that it does not effectively prevent critical amounts of alkaline vapours from penetrating to the other stages of the preheater.
Another known method is to remove part of the exhaust gases from the kiln before these gases are expelled through the kiln inlet, causing the gases thus removed to bypass the preheater and to go directly to an independent cleaning process. Together with the exhaust gases a corresponding amount of alkaline vapours are removed, and these alkaline vapours are thereby kept fully away from the preheater. For example, the alkali vapours may be subjected to an independent condensation process before they are returned as alkaline dust to the kiln or they are passed to a filter and/or valve gate. However, this method suffers from the drawback that the full effect of the preheater is not utilized because all the exhaust gases are not fed to the preheater.
A third method is known from British patent specification No. 1,187,036, in which alkaline dust is removed from a point near the inlet of the rotary kiln. In this process, the seal between the rotary kiln and the smoke chamber or the riser pipe at the inlet end is utilized such that a minor amount of false air is drawn into the smoke chamber through the seal and used for cooling subsequent condensation of the alkaline vapours contained in the exhaust gases. A part of the alkaline dust may then be removed through valve gates or a dust outlet directly beneath the inlet of the rotary kiln so that it is not passed up into the preheater. However, this method does not ensure that no alkali vapours are passed up into the preheater. Thus cakings and cloggings are not prevented.
Other methods of eliminating alkalies in such cement producing processes are disclosed in British Patent No. 1,322,313; U.S. Pat. No. 3,212,764 to Muller et al. (which corresponds to German Patent Publication No. 1,126,306); and U.S. Pat. No. 3,547,417 to Elkjaer. While these developments have improved the process of elimination of alkalies from cement producing systems, they have not proven to satisfactorily eliminate the alkalies while avoiding the drawbacks previously described. I have invented a method and a plant which increase the possibilities of condensing the alkali vapours directly at the inlet of a rotary kiln, such that the alkalies are converted into solids as dust that can be passed out of the system without involving cakings in the system.